This invention relates to a composite electrolyte comprising lithium ion conductive glass-ceramic powder. This invention relates also to a lithium secondary cell comprising this composite electrolyte.
An aqueous or non-aqueous electrolytic solution has generally been employed as an electrolyte for an electric cell. A lithium secondary cell employing a polymer electrolyte made of a polymer material instead of the conventional electrolytic solution has recently drawn attention of the industry. In this type of lithium secondary cell using the polymer electrolyte, electrolytic solution is held in a polymer electrolyte and, therefore, this type of lithium secondary cell is advantageous in that leakage of electrolytic solution as well as corrosion are prevented and, moreover, the structure of the cell is simple and assembling of the cell is easy.
Since lithium ion conductivity of the polymer electrolyte is lower than the electrolytic solution, efforts have been made to reduce thickness of the polymer electrolyte. Reduction in thickness of the polymer electrolyte, however, causes reduction in mechanical strength of the polymer electrolyte with the result that the polymer electrolyte tends to be damaged in the process of production thereby causing short-circuiting of a positive electrode with a negative electrode.
It has, therefore, been proposed, as in Japanese Patent Application Laid-open Publication No. Hei 6-140052 for example, to provide a composite electrolyte by adding an inorganic oxide such as alumina in an electrolyte and thereby increase the mechanical strength. Besides alumina, inorganic oxides such as silica and lithium aluminate are also proposed for addition.
The addition of an inorganic oxide such as alumina to the electrolyte, however, has the disadvantage that lithium ion conductivity in the composite electrolyte is significantly reduced. Besides, when charging and discharging are repeated in a lithium secondary cell comprising this composite electrolyte, reaction between the electrolyte and the inorganic oxide takes place with the result that charging-discharging cycle characteristics in the lithium secondary cell are significantly deteriorated.
It is, therefore, an object of the invention to provide an electrolyte having a medium impregnated with a non-aqueous electrolytic solution which has a high ion conductivity and has sufficient mechanical strength even if thickness of the electrolyte is reduced.
It is another object of the invention to provide a lithium secondary cell comprising a separator made of an electrolyte impregnated with a non-aqueous electrolytic solution which has a high cell capacity, improved charging-discharging characteristics and a long, stable cell life.
Numerous experiments made by the inventors of the present invention have resulted in the finding, which has led to the present invention, that a glass-ceramic composite electrolyte comprising a polymer medium in which glass-ceramic powder of a specific composition and a non-aqueous electrolytic solution are dispersed has a remarkably higher lithium ion conductivity than the prior art composite electrolyte containing an inorganic oxide which does not exhibit lithium ion conductivity. It has also been found that, by applying the composite electrolyte thus obtained to a lithium secondary cell, the cell capacity can be increased and the charging-discharging characteristics can be remarkably improved as compared to a case where the prior art composite electrolyte containing an inorganic oxide which does not exhibit lithium ion conductivity is applied to the lithium secondary cell.
For achieving the object of the invention, there is provided a glass-ceramic composite electrolyte comprising a medium containing glass-ceramic powder impregnated with a non-aqueous electrolytic solution.
According to the invention, the composite electrolyte contains the lithium ion conductive glass-ceramic powder in the lithium ion conductive electrolyte and, therefore, lithium ion conductivity in the electrolyte is not substantially reduced while the mechanical strength of the electrolyte is improved and the cell capacity, particularly charging characteristic, of a lithium secondary cell using this composite electrolyte is increased.
Further, since reactivity of the glass-ceramic powder is very low, reaction between the glass-ceramic powder and the electrolytic solution during charging and discharging hardly takes place and, therefore, reduction in the charging-discharging characteristics of a lithium secondary cell due to reaction between an inorganic oxide such as alumina and an electrolyte as in the prior art composite electrolyte can be prevented.
In one aspect of the invention, the glass-ceramic powder consists of grains having an average grain diameter of 20 xcexcm or below (calculated on the basis of volume), a maximum grain diameter of 44 xcexcm or below and lithium ion conductivity of 1xc3x9710xe2x88x924Sxc2x7cmxe2x88x921 or over.
In another aspect of the invention, the medium comprises a sheet-like polymer material containing glass-ceramic powder.
In another aspect of the invention, the composite electrolyte has a thickness of 100 xcexcm or below and lithium ion conductivity of 1xc3x9710xe2x88x925Sxc2x7cmxe2x88x921 or over.
In another aspect of the invention, the medium contains 10% to 90% in weight percent of the glass-ceramic powder.
In another aspect of the invention, there is provided a lithium secondary cell having a positive electrode, a negative electrode and a separator wherein said separator comprises the above described composite electrolyte of the invention.
The invention will be described more in detail with reference to the accompanying drawings.